Drilling secondary holes from a main borehole is an established technology, for example in creating sidetracks and multilateral wells for formation evaluation or production enhancement. These multilaterals have diameters comparable to the parent borehole. Because of the stiffness of the assembly used to drill them, they take off from the parent hole at a shallow angle, leaving a large elliptical opening at the junction. If the junction needs to be sealed, this requires a considerable investment in a mechanical junction of some kind, over and above the investment of time and money required to drill the multilateral hole in the first place.
Because of this, a multilateral well is typically drilled for production enhancement or control of some kind, when access to a part of the reservoir far from the main borehole is needed. The technology for accessing the reservoir close to the main borehole is also very well established, namely shaped charge perforators. These open channels into the formation, through the casing and cement, to allow the flow of hydrocarbons into the well or treatment fluids into the formation. The holes in the casing created by shaped charges are not smoothly shaped, but are ragged or rough; this makes them difficult to seal, should this be required. The holes created in the rock are also not smooth, clean or uniformly shaped; they are tapered, have a rough wall, and contain debris, and their dimensions depend on rock strength and many other factors.
Due in large part to the aforesaid characteristics, multilateral holes and shaped charge holes are not suitable for a number of uses which would benefit from the access to the rock formation surrounding the main well bore.